1. Field of the Invention
The invention relates in general to display devices and in particular to scrambling and encrypting devices and methods which prevent a signal "in the clear" at the display device.
2. Description of Related Art
To avoid pirating of video, video is scrambled or encrypted by the provider to protect against unauthorized viewing. The term "scrambling" typically means the altering of an analog signal such that it cannot be displayed in a conventional sense without the proper descrambling operation. Examples of scrambling techniques include but are not limited to sync suppression, active line rotation, and line shuffling. The term "encryption" typically is used to describe the operation of altering a digital sequence usually by multiplying it by a pseudo-random sequence. In order to recover the original signal, a "key" is required. An example of this technique is the Data Encryption Standard (DES).
An example of a typical MPEG display system without encryption is shown in FIG. 1. An example of a typical MPEG display system with encryption is shown in FIG. 2, where it should be noted that an encrypted sequence "BLCFADGIHEKJ" is created.
A video signal is often scrambled or encrypted so that without proper authorization, it is in an unusable form. However, if the consumer premises are granted authorization, then typically at interfaces which are accessible to the consumer, such as at the output of a cable converter box or the "video output" jacks of a receiver, the video signal is descrambled or decrypted and hence is "in the clear" (See FIG. 3). There are many types of display devices available today. There is the generally well known cathode ray tube display devices (CRT), and discrete display devices such as digital light modulators or deformable mirror spatial light modulator (DMD), liquid crystal displays (LCD), and plasma displays. Each of these displays have the problems associated with an unauthorized user simply recording the video "in the clear" from the output of the cable converter box or the "video output" jacks of a television receiver. For ease of description reference will be had to the operation of a LCD.
A liquid crystal display device includes liquid crystal picture elements ("pixels") arrayed in rows and columns. In an LCD display system, the discrete display device is illuminated on a "row" or line basis, due to the matrix addressing scheme typically employed. Hence, a particular row is illuminated and then another row is illuminated until an entire frame is created. For example, in a particular Philips LCD display device there are 480 columns by 480 rows worth of pixels. To load the 480 pixels required for a given row, column drivers are required. The 480 rows are then activated sequentially to create a picture (or a frame).
FIG. 4(a) shows a schematic diagram of an LCD display device. Referring to FIG. 4(a), the display device, which is intended to display video, for example television pictures, includes an active matrix liquid crystal color display panel 10. The panel 10 comprises two spaced, transparent and insulating supporting plates, for example of glass, with twisted nematic liquid crystal disposed therebetween and has a large number of liquid crystal picture elements with associated switching elements and memory elements (e.g. capacitors) arranged in rows and columns which are addressed via first and second sets of crossing address conductors with each picture element being connected with a respective address conductor of each set. The first set comprises row address conductors extending in the row direction. Conductors of the second set extend generally in the column direction and hereinafter will be referred to as column address conductors. Each column address conductor is connected with a respective picture element in each row.
In known active matrix liquid crystal display devices, the row address conductors serve as scanning electrodes and are controlled by a row driver circuit 15, comprising a shift register circuit, which applies a selection signal to each row conductor sequentially in turn during a respective row address period. In synchronism with the selection signals, achieved by means of the timing and control circuit 16, data (video) signals, obtained by sampling a TV line with serial to parallel conversion, are applied to the column address conductors from a column driver circuit 17 connected to the output of a video processing circuit 18 to produce a required display effect from the rows of picture elements as they are scanned. Thus the video information for a single line is loaded into the column driver circuit 17 and depending on the row addressed by the row driver circuit 15, this row is loaded with video information. The individual display effects of the picture elements, addressed one row at a time, combine to build up a complete picture in one field, the picture elements being addressed again in a subsequent field. This sequential loading of the array 10 provides a signal "in the clear" which is easily pirated.